The present invention generally relates to the treatment of genetic and metabolic brain disorders, and more particularly to a method for treating genetic and neurodegenerative diseases involving the delivery of corrective genetic materials into the brain.
A large variety of genetic brain disorders, such as Tay-Sach's disease, Alzheimer's disease and Parkinsonism, have been detected, researched and classified. As early as 1902, Sir Archibald Garrod first recognized genetic enzyme deficiency diseases and classified them as "inborn errors of metabolism." Since that time, extensive research has been conducted on the treatment of genetic brain disorders.
For example, in the last twenty-five years, over 360 specific catabolic enzyme deficiency diseases have been characterized. Recent biochemical and genetic research has identified the causes of more than 120 of these diseases, as discussed in McKusick, V. A., Mendelian Inheritance in Man, John Hopkins University Press, Baltimore, Maryland, 1978.
Most diseases involving genetic enzyme deficiencies are characterized by motor and mental deterioration leading to early death. A major group or neurodegenerative genetic enzyme disorders involves diseases classified as "lysosomal storage diseases". Lysosomes are the principal cite of intracellular digestion. They consist of membrane-encapsulated vesicles containing more than forty acid hydrolases capable of degrading most biologically important macromolecules, as discussed in Dan, R. T. et al., "Lysosomes", Essays in Biochemistry 12:1-40, 1976. Lysosomal storage diseases result from either the deficiency or nonfunctionality of one or more of the lysosomal hydrolases. These disorders are further characterized by the accumulation of the glycosphingolipid or glycosoaminoglycan substrate of the deficient enzyme in the lysosome.
A wide variety of neurodegenerative lysosomal storage diseases exist, some of which are listed below in Table I.
TABLE I ______________________________________ Disease Enzyme Deficiency ______________________________________ SPHINGOLIPIDOSES: GM.sub.1 gangliosidosis .beta.-Galactosidase GM.sub.2 gangliosidosis: Classical Tay-Sachs Hexosaminidase A sandhoff's Variant Hexosaminidase A & B AB Variant GM.sub.2 activator Metachromatic Leukodystrophy Arylsulfatase A Krabbe Disease Galactocerebrosidase Fabry Disease .alpha.-galactosidase A Gaucher Disease .beta.-glucosidase Niemann-Pick Sphingomyelinase MUCOPOLYSACCHARISOSES: Hurler and/or Scheie .alpha.- Iduronidase Hunter Syndrome Iduronate Sulfatase Sanfilippo Disease: Type A Heparin-N--sulfamidase Type B .alpha.-N--Acetylgluco- saminidase Type C Heparin-N--Acetyl- transferase Type D .alpha.-N--glucosamine- 6-sulfatase Marguio Disease: Type A Galactosamine-.alpha.- sulfate sulfatase Type B .beta.-Galactosidase Maroteaux-lamy Arylsulfatase B Sly Disease .beta.-Glucuronidase DiFerrante Glucosamine-6-sulfate sulfatase ______________________________________
Most of the diseases listed in Table I actually involve several genotypic and phenotypic variations which have been grouped together on the basis of the defective enzyme.
Research involving genetic enzyme deficiencies has had a significant impact on reproductive counseling. It is now possible in many cases to reliably detect carrier heterozygotes as well as prenatally diagnose defective fetuses. Unfortunately, prenatal diagnosis is complicated by the fact that each disease is rare. Thus, at-risk carrier couples are often not identified until after an affected child is diagnosed.
In addition, some enzyme deficiency diseases may affect certain subpopulation groups more than others. For example, it is now possible to detect at an early stage the existence of Tay-Sachs disease in individuals of Ashkenazi Jewish ancestry However, even when effective screening programs for such diseases exist, they may not be used because of moral or religious convictions. Likewise, elective abortion is not always an acceptable consideration for similar reasons.
While carrier detection and prenatal diagnosis has had some impact in minimizing the number of individuals afflicted with genetic enzyme deficiency diseases, many problems still exist. A need therefore exists for an effective therapeutic program to control genetic enzyme deficiencies in patients having these diseases.
A variety of methods have been used to treat patients having genetic enzyme deficiencies, including lysosomal storage diseases. Current research indicates that the etiology of these
U diseases at least partially results from a decreased concentration of the deficient enzyme product, as discussed in Barranger, J. A., "Feasibility of Enzyme Replacement in Brain: An Overview.", Advances in the Treatment of Inborn Errors of Metabolism, John Wiley, London, 1982. Therefore, a possible therapeutic strategy would be to provide enough of the deficient enzyme product to restore its concentration to normal levels
Another possible theory explaining the pathogenesis of lysosomal storage diseases involves accumulation of the substrate of the affected enzyme Thus, alternative therapeutic strategies might include methods of reducing the concentration of excess enzyme substrate. Methods of accomplishing this involve dietary therapy and chelation of stored metabolites, as well as other methods which decrease the synthesis of the substrate by metabolic inhibition. While these methods have met with success in treating certain diseases (e.g., phenylketonuria), a large number of diseases remain which are not amenable to this type of treatment. In certain diseases, the excess substrate is an essential metabolite that is not readily regulated, and is synthesized throughout the body.
The treatment of genetic lysosomal storage diseases must therefore be approached from a dual standpoint: (1) the control of excess substrate accumulation: and (2) increasing the level of deficient enzymes. To accomplish these goals, a third category of therapeutic treatment may be possible which is called "enzyme replacement therapy." Using this method of treatment, properly administered exogenous enzymes gain access to substrate-engorged lysosomes via fusion with a primary lysosome. The administered enzyme can then restore the normal catabolic function of the affected lysosome. However, there are several problems associated with enzyme replacement therapy, described as follows:
1 Enzyme Delivery Problems
A major problem involving the delivery of exogenous enzymes is that of the blood brain barrier (BBB). The BBB is a capillary barrier comprising a continuous layer of endothelial cells which are tightly bound. The BBB excludes molecules in the blood from entering Z5 the brain on the basis of both molecular weight and lipid solubility, as described in Neuwelt, E. A. et al, "Is There A Therapeutic Role For Blood-Brain Barrier Disruption? ", Ann. Int. Med. 93:137-139, 1980: Rapoport, S. I., Blood-Brain Barrier in Physiology and Medicine, Raven Press, N.Y. 1976. For example, the BBB normally excludes molecules with a molecular weight greater than 180 daltons. Similar exclusion occurs on the basis of lipid solubility.
One method of passing agents through the BBB involves osmotic disruption of the barrier by the administration of hypertonic mannitol or other agents, as described in Neuwelt, E. A., "Osmotic Blood-Brain Barrier Modification: Monoclonal Antibody, Albumin, and Methotrexate Delivery to Cerebrospinal Fluid and Brain", Neurosurgery, 17:419-423, 1985: Rapoport, supra. Disruption of the BBB using this method is caused by a shrinkage of the cerebrovascular endothelial cells, which increases the permeability of the interendothelial junctions.
However, numerous problems exist when exogenous enzymes are administered. Tests indicate that low amounts of enzymes are actually delivered, since organ-derived, purified lysosomal enzymes injected into a patient's blood stream are rapidly cleared, with a half life in the range of several minutes as described in Ratazzi, N., "Enzyme Therapy in Lysosomal Storage Diseases: Current Approaches.", Human Genetics - Part B: Medical Aspects 573-587, 1982. Organ biopsies and radioimmunodiffusion assays demonstrate that the exogenous enzyme is found mainly in the liver, with only minimal activity detectable in extrahepatic tissues. This rapid clearance is most likely caused by hepatic receptors which recognize terminal mannosyl and N-acetyl- glucosaminyl residues on the lysosomal enzymes.
Another problem involving the direct delivery of Z5 exogenous enzymes is the likelihood that a recognition marker will be required for proper enzyme uptake, as indicated in Hickman, S. et al, "A Recognition Marker Required For Uptake of Lysosomal Enzyme By Cultured Fibroblasts.", Biochem.Byophys. Res.Comm. 57:55-61, 1974. Accordingly, effective enzyme replacement therapies will not only require administration of a highly stable, very specific enzyme which is protected from hepatic clearance, but the enzyme must also bind to a receptor with a high affinity that will deliver the enzyme to the proper intracellular compartment.
2.Problems Involving the Availability of Enzyme Supplies
Many of the needed enzymes required for effective enzymatic therapy are difficult and costly to obtain. Likewise, such enzymes frequently must be administered at numerous intervals, requiring substantial amounts of materials to be obtained. It may also take months or even years of enzyme administration for the treatment to be effective.
3. Problems Involving Protection of the Enzyme
In addition to protecting the administered enzymes from rapid renal clearance, the enzymes must also be protected from the patient's immune system, as described in Poznanski, M. J., "Enzyme-Protein Conjugates: New Possibilities For Enzyme Therapy", Pharmac. Ther. 21:5-76, 1983. Adverse immunological responses are particularly evident when the administered enzymes are derived from fungal or bacterial sources. Also, there is the possibility of acute enzyme toxicity caused by the administration of large doses of enzymes. This toxicity may be manifested in acute hyperproteinemia.
Thus, there are numerous problems associated with the direct administration of purified exogenous enzymes in the treatment of lysosomal storage diseases. There have been human Z5 trials involving this method, all of which have met with minimal success. A key problem remaining in exogenous enzyme replacement therapy is the delivery of enzymes across the blood brain barrier, as described above. In order to overcome this problem, another treatment method has been tested which involves direct tissue transplantation.
The first use of organ replacement therapy in a genetic storage disease involved a spleen allograft in a patient having Gaucher's disease, as described in Groth, C. G. et al "Splenic Transplantation In a Case of Gaucher's Disease", Lancet 1260-1264, 1971. However, no clinical improvement in the patient was noted, and death occurred several months following a severe tissue incompatibility response Other tests were conducted involving kidney grafts and liver transplantations, all of which were minimally successful.
Thus, organ and tissue transplantation has not been effective in treating lysosomal storage diseases that affect the CNS. Transplanted organs do not appear to synthesize and/or release sufficient quantities of enzymes in order to control the disease. Even if the transplantation did result in circulation of sufficient amounts of enzymes as has been the case with bone marrow transplants in some of the mucopolysaccharidoses, there is still the problem of delivery across the BBB.
A need therefore exists for a treatment therapy effective in controlling the effects of genetic enzyme deficiency diseases and other genetic and metabolic brain disorders, including Parkinsonism and Alzheime's disease. For example, in Parkinsonism, there is a deficiency of dopamine which may benefit from increased levels of the enzyme tyrosine hydroxylase. There is also some evidence that in Alzheimers's disease there is a deficiency of choline acetyl transferase (CAT). Finally, there is a need for a treatment therapy which Z5 minimizes the problems associated with traditional treatment methods. These problems include transport across the BBB, adverse immunological responses, rapid renal clearance and other physiological difficulties.